1. Field of the Invention
The invention is generally related to the field of cleaning metallic components. More specifically, the invention relates to cleaning metallic arrays, more particularly metallic arrays in electronic components such as integrated circuit flip-chip packages, and other packages.
2. Related Art
In the manufacture of integrated circuits and their various packages, residues are sometimes left from the manufacturing process on metallic components, or are generated prior to or during storage of the products. If the metallic components of the product have a residue from the manufacturing process, or contain residue generated after the manufacturing process while waiting for use, there exists the possibility that the metallic components of the product may not properly perform their intended functions.
Aside from performance, the appearance of the product may not be what is desired, or presumed by the user, due to instability of the product in the presence of the residue. For example, the electrical performance may not be a problem, but a discolored appearance may be enough for a customer to reject the product. Therefore, it has become imperative to find a way to clean discolored metallic arrays to remove residues that might have these deleterious effects.
As well stated in U.S. Pat. No. 5,759,285, a myriad of solder structures has been proposed for the interconnection of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic pads exposed on the surface of a first electronic structure or substrate. A stencil printing operation is used to align the contact mask to the pads. The solder paste areas on the screened substrate are then aligned to corresponding pads on the electronic structure or board to be connected thereto. After alignment, the substrate and board go through a reflow operation to melt the solder paste and create a solder bond between the corresponding pads on the substrate and board.
The ""285 patent describes other known interconnect technologies that use solder balls rather than a solder paste to provide the solder connecting structures. By using plated or evaporated solder balls, a more exact and somewhat greater quantity of solder can be applied than through screening. The solder balls are aligned and are held to a substrate and melted to form a solder joint on a conductive pad of the substrate. The use of solder ball connectors has been applied to the mounting of integrated circuit chips using the so-called C4 (control collapse chip connection) technology since the method and structure were first described and patented in U.S. Pat. Nos. 3,401,126 and 3,429,042. More recently, larger solder balls have been used in surface mount technology to attach single or multichip packages to circuit cards in so-called ball grid array or BGA technology. Regardless of the form of the solder connection or the method of making the solder connection, there are typically three stages at which cleaning of the solder surface may be essential. First, during deployment of the solder prior to making the connections, processing of solder may leave undesirable residues. For example, in evaporating or plating of solder balls for flip-chip (C4) connection or BGA module, photoresist or plating bath residues may be left which will interfere with proper solder wetting of the pads. Second, to maintain the alignment of the solder to the pads on the substrate or card and to allow good wetting of the solder on the pads, flux is most often used and will need to be removed to avoid leaving corrosive contaminants on the packaging assembly. Finally, rework of part-good assemblies requires special handling, which may require a cleaning step that assures the reliability of the package. For example, removal of a reworkable epoxy underfill, as described in U.S. Pat. No. 5,512,613, which is assigned to the present assignee, requires a cleaning process, which does not attack the C4 solder joints. Unfortunately, however, removal of the flux and flux products and cleaning of the solder is a difficult task because the cleaning process may itself be corrosive to the solder and/or electronic component. Additionally, dissolution of the solder can occur resulting in a smaller amount of solder forming the solder bond and cause disposal problems since the solders are generally lead/tin alloys and their solutions pose an environmental threat if discharged.
The solution to many of the problems evidenced in the ""285 patent are achieved by a composition and method for cleaning solder to remove flux, flux reaction products, contaminants, residues from manufacturing operations, e.g., plating bath and photoresist residues and the like without any significant dissolution of the solder. The composition comprises a solution of a non-aromatic sulfonic acid and a substituted alcohol preferably a substituted aliphatic alcohol. The non-aromatic organic sulfonic acid is preferably methanesulfonic acid (MSA) and the alcohol 2,2,2-trifluoroethanol. The method of the ""285 patent uses the above composition by applying the composition to the solder to be cleaned in any suitable way, e.g., by immersion in a bath at a temperature of about 25 to 75xc2x0 C., or higher, preferably 50 to 70xc2x0 C. for 1 to 120 minutes, preferably 5 to 60 minutes. Immersion may be performed under an inert cover (e.g., N2) but this is apparently not necessary.
U.S. Pat. No. 6,250,318 describes the art of cleaning using conventional washing equipment. The equipment generally consists of multiple zones of treatment; namely, a wash zone, a rinse zone and a dry zone. The wash zone typically contains a prewash section, a wash section and an air isolation section. In addition, there is a rinse section that typically includes a first rinse section to wash away the chemical from the wash section, a rinse section, a final rinse section and an isolation section. The final drying section generally contains an air isolation section, and the heated air-dry section. A central exhaust system located at the top of the entire apparatus acts to extract fumes generated from the washing actions, which are extracted by the central exhaust blower external to the cleaning apparatus. There is also a conveyor transport system that runs across the entire length of the various zones of treatment. This conveyor carries the device to be cleaned and moves the devices through the various treatment zones for cleaning and drying. The patent mentions that conventional cleaning equipment in the art generally include commercial spray nozzles attached to a spray manifold or hose, and held across the path of the device to be washed and cleaned. These nozzles are usually very large and bulky in diameter and provide various types of spray cones and angles of discharge. When these nozzles are attached a cross the spray manifold or hose, they cannot achieve close pitching when attached in a straight line. Thus spray angle coverage is the only means by which these spray nozzles reach the entire surface of the devices to be cleaned. This spray angulation causes the edges of the devices to experience a decrease in fluid energy as in the decrease of mass momentum of fluid motion. Advancement in miniaturization of semiconductor chips with the creation of micro-ball grid area packages and chip scale packages has seen the input-output interconnect leads being replaced by solder balls, with a pitch of around 1.0 to 1.5 mm. With the bulky size of the spray nozzles, cleaning of such closely packed devices become ineffective.
U.S. Pat. No. 6,203,637 describes use of a cleaning process. During the so-called xe2x80x9cpackagingxe2x80x9d of integrated circuits, several operations are distinguished which comprise a bonding of surfaces: 1. The individual integrated circuits are cut out of a silicon wafer and are applied to semiconductor system carriers and are bonded with these (so-called die bonding). The semiconductor system carrier surface is normally made of copper or of nickel, silver or gold or of a material on an epoxy basis, generally of a plastic material. Examples of such semiconductor system carriers are punched or etched metallic lead frames, ceramic substrates or ball grid array substrate carriers made of plastic. Hard soldering, soft soldering and gluing are used as bonding processes. In the case of flip chip solder processes, the integrated circuit is applied to a semiconductor system carrier by geometrically separated solder balls, which are simultaneously used as an I/O-bonding. 2. Bonding the integrated circuits with contact support points on the semiconductor system carrier, as, for example, on the lead frame. The participating surfaces are metallic, for example, of Al, Au, Cu, Ni, Pd. Here, soldering or welding, particularly flux-free soldering or ultrasonic welding, are used as bonding techniques. This step is known as wire bonding. 3. Molding: In this process step, the circuits are molded by a molding material on the semiconductor system carriers, for example, the lead frames, after the wire bonding, the above-mentioned surfaces of the semiconductor system carriers and the integrated circuits participating with respect to the molding material. This patent mentions as a solution to the bonding problem the recognition that if gas plasma cleaning processes can proceed in an atmosphere, preferably in a vacuum atmosphere, with activated hydrogen, this will always lead to a xe2x80x9cconservationxe2x80x9d of the surfaces with respect to air. In other words the invention even permits the storing of the surfaces after the xe2x80x9ccleaningxe2x80x9d and before their bonding, in air, without the occurrence of the disadvantages with respect to the capacity to be bonded.
Other patents to be mentioned include U.S. Pat. Nos. 5,532,094; 5,593,504; 5,821,208; 5,910,010; and 6,229,215.
To the best knowledge of the inventors herein, the discoloration of gold arrays on flip chip ball grid array (FCBGA) has not been publicly disclosed, and therefore, no cleaning methodologies have been proposed for this particular problem. While the root cause of the discoloration is still under investigation, the inventors herein believe that it results from surface contamination by organic and ionic species, which get transported from the back surface of the assembly to the front surface during a metrology step that involves immersion of the assemblies in an aqueous solution. FIG. 1 is a photograph of a discolored gold surface of a FCBGA assembly. As may be seen, the discoloration is in the form of circular spots or irregular patches, spread in an uneven fashion on the gold surface.
In a FCBGA, the solder bumped chip is placed face down onto matching wettable bonding pads on a multilayer BGA substrate. And the assembly is reflowed. The bonding pads on the substrate may initially be bare of any solder or may be solder bumped and coined prior to chip assembly. After flip chip attachment, the chip is under-filled with an epoxy resin formulated to relieve the stresses induced by the thermal mismatch between the chip and the substrate and to prevent moisture form getting to the chip surface. After chip assembly, an adhesive is applied to the top surface of the stiffener ring (typically made of copper), and the back of the chip is covered with a pliable conductive adhesive. A copper plate is attached to the stiffener ring, while the center of the plate is in contact with the adhesive on the back of the chip. The heat dissipated by the chip is thus conducted through the adhesive to the heat spreading copper plate. A heat sink can be attached to the plate to further lower the case to ambient air thermal resistance. The exposed metal surface finishes on the substrate may be electroplated or electroless plated with Ni/Au or coated with an organic surface preservative. After chip encapsulation, solder balls are attached to the underside of the substrate to complete the FCBGA. FIG. 2 is a cross-section of a typical FCBGA assembly.
In addition to the discoloration problem, it would be preferable if a cleaning methodology could be developed that avoids immersion and the use of chemicals that might have an adverse affect on the entire packaging that is often comprised of a variety of materials (ceramic, gold, copper, plastic and the like). Highly concentrated fluids and some organic solvents might be more able cleaners, but may be too aggressive and pose environmental and other regulatory risks. In addition, after the discoloration is removed, the chemical is preferably removable so that no residual chemical resides on the surface. From the above it is clear that there is currently no acceptable way to remove discoloration from metallic surfaces of BGA packages without the use of exotic plasmas and other means. It would be advantageous if methods could be provided which address the need for non-discolored ball grid array packages.
In accordance with the present invention, discoloration on exposed metal surfaces of ball grid array packaging and other electronic packages may be cleaned by a method comprising a combination of an organic cleaning step with an aqueous acidic chemical cleaning step, followed by thoroughly rinsing the cleaned surface with deionized water and preferably subsequent drying of the surface. The organic cleanup step removes nonionic residues and prepares the surface for the second cleaning step that involves acidic reagents for chemical dissolution of oxides, hydroxides and other species on the surface of the metal.
The inventive methods of removing discoloration from a metal surface (preferably gold, nickel, or alloy of gold and nickel) of an electronic device (preferably a FCBGA), comprise the steps of:
a) exposing a metallic surface of an electronic device to a first composition comprising an organic reagent (preferably high purity acetone diluted about 1 to about 10 times with deionized water), the metallic surface having discoloration thereon, under conditions sufficient to form a first intermediate metallic surface substantially devoid of non-ionic residues;
b) contacting the first intermediate metallic surface with a second composition comprising an acid (preferably hydrochloric acid diluted about 1 to about 50 times with deionized water) under conditions sufficient to form a second intermediate metallic surface substantially devoid of non-ionic residues, oxides, hydroxides and the like; and
c) rinsing the second intermediate metallic surface with deionized water, the deionized water substantially devoid of organic compounds, under conditions sufficient to form a cleaned metallic surface substantially devoid of non-ionic residues, oxides, hydroxides, organic residues, the first composition and the second composition.
Preferably, the method further includes the step of drying the cleaned metallic surface employing either a contact drying method, a non-contact method, or combination thereof.
Preferably, said drying by non-contact method comprises contacting the integrated circuit flip-chip package with a gas selected from the group consisting of nitrogen, argon, helium, hydrogen, oxygen, a halogen, and mixtures thereof.
Preferably, said other means are selected from the group consisting of gas purging, heating, baking, vacuum, and combinations of same.
During steps a) and b), the conditions are sufficient ether to i) remove the residue; or ii) chemically modify the residue to form a modified residue that may be removed by other means, for example, the preferred rinsing and drying steps.
Another aspect of the invention is a method of removing discoloration from a metal surface (preferably gold, nickel, or alloy of gold and nickel) of a flip-chip ball grid array, the method comprising the steps of:
a) exposing a metallic surface of the flip-chip ball grid array to a first composition comprising high purity acetone diluted about 1 to about 10 times with deionized water, the metallic surface having discoloration thereon, under conditions sufficient to form a first intermediate metallic surface substantially devoid of non-ionic residues;
b) contacting the first intermediate metallic surface with a second composition comprising hydrochloric acid diluted about 1 to about 50 times with deionized water under conditions sufficient to form a second intermediate metallic surface substantially devoid of nonionic residues, oxides, hydroxides and the like; and
c) rinsing the second intermediate metallic surface with deionized water, the deionized water substantially devoid of organic compounds, under conditions sufficient to form a cleaned metallic surface substantially devoid of non-ionic residues, oxides, hydroxides, organic residues, the first composition and the second composition; and
d) drying the cleaned metallic surface employing a drying method selected from the group consisting of contact methods, non-contact methods, and combinations thereof.
Further understanding of the invention will be forthcoming after review of the description of preferred embodiments and claims that follow.